This invention relates in general to vehicle drum brake assemblies and in particular to an improved structure for a composite brake drum for use in such a vehicle drum brake assembly and method for producing the same.
Most vehicles are equipped with a brake system for slowing or stopping movement of the vehicle in a controlled manner. A typical brake system includes either a disc brake assembly or a drum brake assembly for each of the wheels. The brake assemblies are typically actuated by hydraulic or pneumatic pressure generated by an operator of the vehicle depressing a foot pedal, pulling a hand lever, and the like. The structure and operation of the brake assemblies, as well as the actuators therefor, are well known in the art.
FIG. 1 illustrates a prior art pneumatically actuated drum brake assembly, indicated generally at 10, typically for use with a heavy duty truck and trailer. As shown therein, the drum brake assembly 10 includes a backing plate 12 which is secured to a fixed, non-rotatable component of the vehicle, such as the vehicle axle housing (not shown). A pair of opposed arcuate brake shoes 14 (only one brake shoe 14 is illustrated) are supported on the backing plate 12 for selective movement relative thereto. Each of the brake shoes 14 has a friction pad 16 secured thereto.
The brake drum assembly 10 further includes a hollow cylindrical "heavy duty" composite brake drum 18 shown in prior art FIG. 2. The brake drum 18 is disposed adjacent the backing plate 12 such that the brake shoes 14 extend within an inner cylindrical braking surface 24A thereof. To effect braking action, the brake shoes 14 are moved outwardly apart from one another such that the friction pads 16 frictionally engage the cylindrical braking surface 24A of the brake drum 18. Such frictional engagement causes slowing or stopping of the rotational movement of the brake drum 18 and, therefore, the wheel of the vehicle in a controlled manner.
One or more actuating mechanisms are provided in the drum brake assembly 10 for selectively moving the brake shoes 14 outwardly apart from one another into frictional engagement with the cylindrical braking surface 24A of the brake drum 18. Usually, a pneumatically actuated service brake mechanism is provided for selectively actuating the drum brake assembly 10 under normal operating conditions. Typically, the service brake mechanism includes an air chamber device 26, a lever assembly 28, and a S-cam actuating mechanism 30. To actuate the service brake, pressurized air is supplied to the air chamber device 26 to actuate the lever assembly 28 which in turn rotates the S-cam actuating mechanism 30 to move brake shoes 14 apart from one another into frictional engagement with the cylindrical braking surface 24A of the brake drum 18. A mechanically actuated parking and emergency brake mechanism is also usual provided for selectively actuating the drum brake assembly 10 in a similar manner.
FIG. 3 illustrates a typical sequence of steps for producing the brake drum 18 shown in prior art FIGS. 1 and 2. First, referring to the left hand side of FIG. 3, the steps involved in the process of forming a shell portion 22 and a liner portion 24 of the brake drum 18 are illustrated. Initially, in step 40, a flat sheet of suitable material, such as for example steel, is formed into a generally flat band having a desired profile, such as by a roll forming process. Next, in step 42, the opposed ends of the band are disposed adjacent one another and welded together to form a hoop. In step 44, the hoop is expanded to produce the shell portion 22 having a desired profile shown in FIG. 2. Following this, the liner portion 24 is cast in the shell portion 22, preferably by a centrifugally casting process in step 46. After this, in step 48, the shell portion 22 and the liner portion 24 are rough machined.
Now, referring to the right hand side of FIG. 3, the steps involved in the process of forming a mounting flange portion 20 of the brake drum 18, and the steps involved in the process of forming the brake drum 18 itself, are illustrated. In step 50, a flat sheet of suitable material, such as for example steel, is formed into a mounting flange blank, such as by a stamping process. Following this, in step 52, the mounting flange blank is formed into the mounting flange portion 20 having a desired profile by a stamping process. A. plurality of lug bolt mounting holes 20C (only one lug bolt mounting hole 20C being illustrated in FIG. 2), can be simultaneously formed in the flange portion 20. As is known, lug bolts (not shown) extend through the lug bolt holes 20C to secure the brake drum 18 to a vehicle wheel (not shown) for rotation therewith. In step 54, an inner end 20A of the mounting flange portion 20 is disposed adjacent an outer end 22B of the shell portion 22 and welded together to join the shell portion 22 and the liner 24 portion to the mounting flange portion 20. Next, a pilot hole 20B is formed in the mounting flange portion 20 during step 56.
In step 58, the brake drum 18 is finish machined to predetermined tolerances. Following this, the brake drum 18 is typically subjected to a balancing operation in step 60. In particular, one or more wheel balance weights (not shown) are usually attached to an outer surface of the shell portion 22 by welding to produce the finished brake drum 18. Typically, the mounting flange 20 of the brake drum 18 defines a generally constant mounting flange thickness T1, and the shell portion 22 defines a generally constant shell thickness T2 which is less than the mounting flange thickness T1. Alternatively, the brake drum can be a heavy duty "full cast" brake drum, indicated generally at 32 in prior art FIG. 4. As shown therein, the brake drum 32 includes an integral raised squealer band 34 provided on an outer surface thereof.
The composite brake drum 18 illustrated in FIGS. 1 and 2 is considerably lighter than the full cast brake drum 32 illustrated in FIG. 4. However, the full cast brake drum 32 can be produced using a simpler manufacturing process than the process used to produce the composite brake drum 18. Also, each of the brake drums 18 and 32 typically incorporates a sufficient imbalance which renders them unsatisfactory for use on a vehicle without balancing. There are several known methods for correcting the imbalance of the brake drums 18 and 32. Typically, the composite brake drum 18 is balanced by welding balance weights to the outer surface of the drum. While the full cast brake drum 32 can be balanced in a similar manner, it can also be balanced according to the methods disclosed in U.S. Pat. No. 4,986,149 to Carmel et al. and U.S. Pat. No. 5,483,855 to Julow et al. According to the method of the Carmel et al. patent, a crescent or wedge of material is preferably cut away from an outer surface of the squealer band by a lathe during an eccentric turning process to produce a final balanced brake drum. According to the method of the Julow et al. patent, a circumferentially extending substantially constant depth cut is made along a portion of the squealer band by a milling machine to produce a final balanced brake drum. Thus, it would be desirable to provide an improved structure for a composite brake drum and method for producing such a brake drum which is relatively simple and economical.